Where electrical devices are employed in explosive atmospheres precautions must be taken to avoid igniting those atmospheres by reason of an arc produced by an electrical component such as a switch, a circuit breaker, etc. A conventional procedure is to mount such electrical devices within an enclosure sufficiently rigid to withstand the force of an explosion occurring within the container and thereby preventing the flame occurring within the enclosure from reaching the exterior thereof. Such housings may not be completely fluid tight but may have small passageways which serve as vents permitting the escape of gases while being sufficiently small to prevent the escape of flame. Since the passageways are small there can be a substantial pressure build-up in the housing. This necessitates that the housing structure be quite rigid. Such housings often comprise a metal casting. These are quite expensive to manufacture.
The present invention contemplates the use of a plastic housing which has the advantage of being substantially less expensive than a metal casting. Furthermore, the plastic is an electrical insulator so that the electrical components within the housing can be mounted directly on the interior portions of the housing, without the necessity of extra parts to serve to insulate the electrical components from the housing walls, as in the case of a metal housing. To compensate for the difference in strength between metal walls and plastic walls for the housing, a relatively large venting passageway is provided between the interior of the housing and the exterior, the passageway being sufficiently large in area to avoid substantial pressure build-up within the housing. At the same time, the passageway is sufficiently long and narrow as to serve to prevent any flame from within the housing propagating to the exterior of the housing through the passageway. That is, the plastic walls defining the sides of the passageway will cool the burning gases to the point where they are extinguished before reaching the exterior of the housing.
Further objects and advantages will become apparent from the following description taken in conjunction with the drawings.